Intracardiac leads are implanted in a myocardium cavity of a patient and collect depolarization signals for continuously monitoring a patient's cardiac rhythm and, if necessary, for applying electrical stimulation, resynchronization, cardioversion and/or defibrillation pulses. These leads are coupled at the proximal end to an “active implantable medical device” as defined by Directive 90/385/EEC of 20 Jun. 1990 of the Council of European Communities.
Generally, a stimulation or defibrillation lead includes a “lead body” formed by a sheath or a tube (e.g., a flexible hollow tube). The lead body bears an electrode or electrodes that come into contact with the patient's myocardium, and a retractable screw. At its proximal end, the lead body is connected to a connection plug electrically connected to a connector header of a pacemaker or a defibrillator. The tube of the lead body contains one or more conductors electrically connecting the terminals of the proximal connection plug to the electrode or electrodes of the distal head.
For a lead body having a retractable screw, the lead head is equipped with a mechanism to retract the screw to protect the walls of the vein, and to avoid inadvertent hooking on the tricuspid valve or its Chordae tendineas during the introduction of the lead through the vein until the terminal end of the lead head comes to a stop against the wall of the endocardium. The retractable screw system also protects the screw from any deformation that would render it ineffective.
Once the desired position is reached, the surgeon manipulates the lead and anchors it via a simultaneous double movement of axial translation, first deploying the screw out of the housing of the lead head, and second rotating the screw to achieve its anchorage in the wall of the endocardium.
The lead is normally of a pin-driven type. The surgeon holds in one hand the proximal end of the lead body and turns with the other hand, directly or through the intermediary of a tool, a pin at the proximal end. Specifically, the pin is secured to an axial conductor extending within the lead body, and this conductor is free in rotation and is connected at its distal end to the deployment mechanism of the screw.
This technique assumes that the configuration of integrated conductors inside the lead body is of a “coaxial” type, for example, with a central conductor that is used to transfer the rotational torque to the mechanism for deploying the screw and a coaxial conductor extending around the periphery of, but interior to, the lead body.
A variant of this screw manipulation, different from rotating the pin connector, concerns using the end of a stylet inserted into the lead body, as described in EP 1 331 021 A1 and its US counterpart U.S. Pat. No. 7,096,071 (both assigned to ELA Medical, now know as Sorin CRM). This technique provides a limiting function for the drive torque of the screw by twisting the internal part of the stylet. The drive torque limitation allows in particular avoiding tissue and/or screw damage, in order to enable several attempts to anchor the lead if the electrical performance is deemed insufficient.
Another technique is proposed by EP 0 591 053 A1 and its US counterpart U.S. Pat. No. 5,447,534 (both assigned to ELA Medical, now know as Sorin CRM) which describe the Stelix™ brand lead currently marketed by Sorin CRM, Clamart, France. To deploy the screw, a surgeon inserts into the lead body a “stylet-screwdriver” type of stylet whose distal end has a flattened shape cooperating with a counterpart body of the screw deployment mechanism inside the lead head. The deployment is achieved by maintaining with one hand the stylet-screwdriver, and by turning with the other hand the lead body (the proximal portion of the sheath) over five to six complete turns. The lead head is equipped with a system to limit the drive torque that minimizes the torsion strain during screwing: if the resisting torque during screwing exceeds a given threshold, the screw is no longer driven by the rotation of lead body.
This system is effective, safe, and reversible. The surgeon can remove and retract the screw by an opposite maneuver to the installation procedure. However, the flexibility of deployment by rotating the lead body goes against the usual pin-driven maneuver which, as explained above, maintains the end of the lead body fixed and rotates the end of the connector pin (or the handle of a stylet emerging from the lead body in the same place).
Another drawback common to the above-described retractable screw devices, is their relatively large dimensions. The standard diameter of these intracardiac leads is about 7 French (1 French=⅓ mm), thus requiring the use of an 8-French introducer. The value of 7 French corresponds to a minimum technical limit with which it is difficult and dangerous to overcome with the “coaxial configuration” type leads described above. Indeed, for reliability, the lead body requires a minimum thickness of insulation and of conductor. Moreover, the retractable mechanism must also be technically manufacturable to keep an “isodiameter” type configuration, in which a lead having the same diameter along the length of the distal part to be implanted in the venous system, specifically to facilitate the implantation (and a subsequent explantation). This means that the outer surface of the lead, including the location of the electrodes and the deployment mechanism of the screw, is required to maintain its diameter with a tight tolerance. These constraints make it difficult to realize these leads, particularly because of the need for a reliable electrical connection between the inner and outer conductors and the corresponding electrodes of the lead head.
To reduce the diameter of the leads, a different configuration of the connecting conductors has been proposed, a configuration known under the names “co-radial” or “with insulated conductors”. A lead realized according to this technology is described in U.S. Pat. No. 5,571,157. In this co-radial configuration, the two (or more) conductors are insulated conductors wound side by side in a region of the peripheral wall of the sheath of the lead body, forming a coil of single radius (hence the name “co-radial”), thus on a single layer, unlike the coaxial configurations involving two (or more) layers to integrate the different conductors. Insofar as all the conductors are arranged in a single layer, it is possible to reduce the standard diameter to 4.8 French, and the lead can be made with a 5-French introducer. This makes the lead as small as that of a monopolar lead, but with the functionalities of a bipolar or multipolar lead. A smaller diameter makes the lead more maneuverable in the venous system during its implementation, and a system with two or more leads can be implemented in a ventricle.
In order to equip such a co-radial lead with a deployable mechanism for fixing screws, the pin-driven maneuver technology is a priori not applicable. Indeed, it seems difficult to run one conductor over another (although this possibility is envisaged by the U.S. Pat. No. 5,716,390), due to the reliability constraints of the electrical connections at both ends of the lead. On the other hand, the limited capacity of wound conductors to transmit torque reduces the effectiveness for deploying the anchor screw.